Evaluation of 1-Hydroxyethane-1,1-diphosphonic Acid and Sodium Carbonate as Stripping Agents for the Removal of Am(III) and Pu(IV) from TRUEX Process Solvent

Abstract

Abstract The TRUEX solvent extraction process is being developed for the removal of the transuranic (TRU) elements from some of the wastes stored in tanks at the U.S. Department of Energy's Hanford Site. Initial flowsheets for this process call for the stripping (back extraction) of TRUs with 0.2 M 1-hydroxyethane-1,1-diphosphonic acid (HEDPA) followed by washing of the process solvent with 0.25 M sodium carbonate. The stripped TRU stream is to be neutralized with caustic for interim storage in carbon steel tanks prior to vitrification. Such a scenario would result in a considerable amount of phosphorus and sodium in the TRU stream, which could preclude this material from being efficiently vitrified in the proposed Hanford Waste Vitrification Plant (HWVP). An examination of alternative stripping processes suggests that the amount of phosphorus and sodium in the feed to HWVP can be reduced by 1) using a less concentrated solution of HEDPA in the strip step, or 2) using sodium carbonate as the stripping agent. The latter approach would eliminate the introduction of phosphate into the TRU waste stream. It would also greatly simplify the process by combining the strip and solvent wash steps. Furthermore, less caustic would be required to prepare vitrification feed streams for interim storage. A third approach involving the use of a combined sodium carbonate/NaHEDPA solution to strip the TRUs also shows promise. The distribution behaviors of Am(III) and Pu(IV) between HEDPA solutions or sodium carbonate solutions and TRUEX solvent have been examined. Results of stripping experiments using solutions prepared from both spiked and actual Hanford tank wastes are presented in this paper.